Eutectics are phenomena of nature. A simple binary eutectic system is typified by the metallic alloys of lead and tin. Pure elemental tin exhibits an equilibrium freezing point of 232.degree. C. and pure elemental lead exhibits an equilibrium freezing point of 327.degree. C. With but one exception, alloys of tin and lead solidify and melt over a temperature range. The temperature at which a lead-tin alloy begins to solidify will be less than the freezing temperature of elemental lead and may also be less than the freezing temperature of elemental tin.
The exception referred to above is an alloy of 38.1 wt.% lead and 61.9 wt.% tin. This alloy is the alloy of the eutectic composition. The eutectic alloy will freeze, under equilibrium conditions, at the eutectic temperature of 183.degree. C. Also, under equilibrium conditions, the solidification of lead-tin alloys of non-eutectic composition will be completed at the eutectic temperature.
Eutectics exhibit a variety of structures. Such terms as lamellar, plate-like, rod-like, discontinuous, and divorced are commonly used to describe the physical appearance of eutectics. The eutectic structure is dependent upon many factors including the components of the alloy system, the nature and quantity of any impurities present, and the rate at which they are formed. For example, the eutectic of an alloy system may exhibit a regular periodic array of lamellae, or plates, when solidified at near equilibrium rates. As the solidification rate increases, the width of the lamellae will generally decrease, the periodicity will generally become more irregular and individual lamella may terminate abruptly or branch into one or more lamella thus creating faults in the otherwise periodic uniform structure. At very rapid rates of solidification, the near-equilibrium lamellar structure may break down completely and form a new structure with a markedly different appearance.
Eutectics are found in metallic, ceramic, and organic systems and need not be formed from elements, i.e., combinations of compounds may form eutectics. Transparent organic eutectics having lamellar microstructures and the method of making them are described, for example, in U.S. Pat. No. 3,484,153 to Hunt and Jackson. A binary eutectic formed from two elements, or compounds, is the simplest eutectic and more complex eutectics, e.g. ternary and quaternary, are also observed.
Eutectics have many unique properties which make them candidates for many structural and non-structural applications. An example of the use of eutectics in a structural context is the report by Bruch et al. in the Proceedings of the Conference on In Situ Composites-III (Ginn Custom Publishing, p. 258, 1979), that the eutectic alloy NiTaC-13 has been directionally solidified in the form of jet engine turbine blades and successfully engine tested. The same Proceedings contain several papers on eutectics for nonstructural applications. The first paper in the series on nonstructural applications is the one at p. 171 by Yue which reviews the use of directionally solidified eutectics for electronic, magnetic, thermomagnetic, and superconducting applications.
Eutectics may be produced in bulk, as exemplified by the turbine blade discussed above, or in thinner sections for nonstructural applications or for academic purposes such as the study for solidification mechanics. Eutectics in bulk form have been grown under unidirectional cooling conditions by such means as the Bridgman, Czochralski, zone levelling or floating zone techniques.
Various techniques have been used previously to produce eutectics in thinner sections. Generally, the prior art techniques produced films that were poor in quality, e.g., were non-uniform in thickness, were not fault-free over large regions, and exhibitedpoor alignment of the lamellae relative to the lateral surfaces.
Albers and Van Hoof report, for example, (Journal of Crystal Growth, 18, p. 147, 1973) use of a modified Czochralski technique to produce films of the Cd-Zn eutectic. Those films were extracted from the melt by immersing a form, such as a wire loop, or a substrate into the melt and slowly withdrawing the form or substrate from the melt. The films of Albers and Van Hoof were not fault-free, appeared non-uniform in thickness, and exhibited an anomalous relationship between interlamellar spacing and pulling rate which is possibly indicative of non-uniform heat flow during the extraction process. Takahashi and Ashinuma (Jnl. of Inst. for Metals, 87, p. 19, 1958-59) used a technique similar to that of Albers and Van Hoof to produce thin films of the Pb-Sn eutectic. The Pb-Sn films produced by Takahashi and Ashinuma were irregular in thickness and were not fault-free over large areas, but were suitable for their purposes which was the study of the eutectic by means of the electron microscope.
Another technique for producing thin eutectic structures is that of Dhindaw et al., reported at page 60 of the above-referenced Proceedings, wherein lead-cadmium and lead-tin eutectic alloys were encapsulated in stainless steel or quartz capillaries. Dhindaw et al. report, inter alia, that as the distance between the walls of the capillary decreased, there was an increased tendency for the lamellae of the eutectics investigated to align perpendicular to the walls at the walls and to form parallel plates aligned perpendicular to the walls in the region between the walls. That behavior was attributed to a constraining effect at the walls. As the distance between the walls increased, the constraining effect at the walls reportedly became less effective in maintaining the perpendicular alignment of the lamellae at the walls resulting in the observed increased non-perpendicularity at the walls. That effect, in turn, caused the lamellae in the region between the walls to form at increased angles to the walls and to exhibit an increased tendency for fault formation.